Gas turbines are widely used in industrial and commercial operations. A typical gas turbine includes an axial compressor at the front, one or more combustors around the middle, and a turbine at the rear. The compressor generally includes alternating stages of circumferentially mounted stator vanes and rotating blades. The stator vanes typically attach to a casing surrounding the compressor, and the rotating blades typically attach to a rotor inside the compressor. Ambient air enters the compressor, and each stage of stator vanes directs the airflow onto the following stage of rotating blades to progressively impart kinetic energy to the working fluid (air) to bring it to a highly energized state. The working fluid exits the compressor and flows to the combustors where it mixes with fuel and ignites to generate combustion gases having a high temperature, pressure, and velocity. The combustion gases exit the combustors and flow to the turbine where they expand to produce work. For example, expansion of the combustion gases in the turbine may rotate a shaft connected to a generator to produce electricity.
During various operating conditions, it may be desirable to adjust the angle of the stator vanes with respect to an axial centerline of the compressor. For example, the stator vanes may be aligned further from the axial centerline of the compressor to suppress the onset of compressor stall at lower rotational speeds associated with start up or shutdown of the compressor. Conversely, the stator vanes may be aligned closer to the axial centerline of the compressor to allow more working fluid to flow through the compressor and increase the power output of the gas turbine during heavy or sudden increases in electrical demand on the generator.
U.S. Pat. Nos. 5,281,087; 6,551,057; and 6,794,766, assigned to the same assignee as the present application, disclose an electromechanical or hydraulic system for varying the position of stator vanes. In each patent, a single actuator connects to multiple stages of stator vanes to vary the position of the stator vanes with respect to the axial centerline of the compressor. However, the length and width of the stator vanes generally decreases along the axial length of the compressor. As a result, the length of travel for both the actuator and the stator vanes varies by stage. In addition, the cumulative manufacturing tolerances associated with both the actuator and the stator vanes increases proportionally as the size of the stator vanes increases. Therefore, the ability to precisely position stator vanes in different stages using a single actuator is difficult, and a system and method for independently varying the position of stator vanes in different stages would be useful.